Note: Descriptions are shown in the official language in which they were submitted.
CA 02307258 2003-05-28
AV-3847
Improved Backup Pad for Rotary Grinder
The present invention relates to backup pads for rotary grinders and
particularly for grinders that permit a view of the surface during grinding.
Rotary grinders comprising a round abrasive disk attached to a backup pad
which is mounted on a spindle rotated by the grinder have traditionally been
used at
an angle of about 30 to 40 degrees to the workpiece being ground such that the
edge
of the disk is essentially the only part of the disk surface that is used.
This is of
course wasteful and the intensity of the grinding pressure exerted on a
relatively small
area means that the disk will be worn out sooner than would otherwise be the
case.
This problem was addressed in a pair of cases directed first to a backup pad
and second to an abrasive disk to be used with the backup pad, (WOlUS96/18927
and
WO/US96/19191 respectively). Thf;se applications dealt with abrasive disks and
matching backup pads in which the objective was to allow the user a view of
the
surface being ground while the grinder was actually in use. This allows the
disk to be
used at a much flatter angle such that more of the disk is in contact with the
work
surface and the pressure is reduced, leading to a longer life. In addition the
design of
the disk produces inte~TUpted al>radi.ng giving time for cooling and swarf
removal
between grinding intervals. ~('he disks therefore cut cooler and this too
contributes to
longer life.
Such disks however have one disadvantage in that, operating at a flat angle in
a confined space, there is a danger that the edge of the disk grinding a
relatively flat
surface could come into contact a portion of the workpiece at an angle to the
surface
being ground and be damaged by such contact. One proposal to minimize this
problem is to ensure a small overlap of the disk over the backup pad such that
the disk
is able to bend to respond to the contact without ripping or losing the
integrity of the
abrasive surface. This approach is disclosed in L1S Patent No. 6,159,089 and
is highly
effective in most cases. In so .rne cases however it is desirable that the
abrasive disk be
provided with some support even in the peripheral regions. This is the case
for
example when the disks are paper or film backed such
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that the disk of its nature does not have the outstanding physical strength of
a fiber-
backed disk.
The present invention overcomes this problem by providing a backup pad for
an abrasive disk with support over the whole surface of the disk but with
resilience at
the critical edge portions.
Description of the Invention
The present invention provides a backup pad for an abrasive disk which
comprises a circular compressible pad having a first surface adapted for
attachment to
an abrasive disk and a second surface laminated to a relatively rigid support
member
having an axially located mounting means for attachment to a rotary grinder,
wherein:
a. the compressible pad is provided with from three to six gaps in the
circumference spaced around the periphery of the pad and penetrating from 10
to 50% of the radius of the pad; and
b. the rigid support member has a similar shape and dimensions to the
compressible pad except that a segment is removed from at least a portion of
the circumference of the support member between the gaps such that the pad
extends radially beyond the support member in the vicinity of such removed
segments.
While the whole of the backup pad has resilience against pressure normal to
the pad, the edge portions which support the portion of an abrasive disk most
susceptible to damage, that is the radial extremities, are able to deform to a
greater
extent where the compressible pad is not backed by the relatively rigid
support
member.
The extent of the overlap of the compressible pad beyond the support member
is generally not greater than about 1 S%, such as for example from 2 to 10%,
of the
greatest radial dimension of the compressible pad. This overlap can be by a
uniform
amount but more frequently the segment of the circumference removed is a chord
segment such that the degree of overlap is greatest at the center of the
segment and
diminishes towards at least one of the ends.
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The gaps in the circumference of the compressible pad and the support
member are similar in shape and extent. They can have any desired dimension
and
can extend towards the center, or axis, of the backup pad to a point that is
as much as
50%, for example from 20 to 45%, of the maximum radius of the backup pad.
The points at which the gaps meet the circumference of the backup pad are
preferably rounded to minimize the danger of the edges catching during
rotation.
Since an abrasive disk and its associated backup pad usually have a design
direction
of rotation, one edge of the gap can be designated the "leading edge" and the
other the
"trailing edge". In such cases it is necessary only to round off the trailing
edge of the
gap. Such "rounding ofd' is preferably more pronounced on the support member
than on the compressible pad. As a result there is an overlap of the
compressible pad
over the support member in the rounded-off area which becomes continuous with
the
overlap as a result of the removal of a segment from the periphery of the
support
member.
The shape of the gap can be angular, such as a V-shape, or more preferably
curved as for example would be the case if the shape was that of a notional
circle with
a portion lying beyond the radius of the backup pad. More preferably however
the
shape of the gap is generally parabolic with the arms of the parabola
diverging
towards the edge of the backup pad. In a particularly preferred form of gap
the shape
is parabolic with the axis of the parabola non-coincident with a radius of the
backup
pad. In this preferred form, the leading edge of the gap is shorter than the
trailing
edge and, in a preferred symmetrical form of this configuration, the axes of
all the
parabolic gaps in the backup pad form a geometric shape with the axis of the
disk at
the geometric center of the shape.
The material from which the compressible pad is made is usually a foamed
rubber such as a polyurethane, but any resilient material can be substituted
depending
on the degree of resilience demanded by the application. The thickness of the
pad is
typically from about 3 to about l5mm and preferably from 5 to l Omm. The
maximum radial dimension is largely determined by the abrasive disk it is
intended to
support and thus backup pads with maximum radial dimensions of from about 7.6
to
17.7 cm are generally preferred.
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One surface of the compressible pad is provided with a means for attachment
of an abrasive disk. This means could include for example a surface adapted to
receive an abrasive disk with a pressure sensitive adhesive backing, an
adhesive
coating adapted to adhere to the back of an abrasive disk, or one half of a
"hook and
loop" attachment mechanism. This latter term is intended to cover all
mechanisms in
which a releasable physical attachment of two components occurs such that
rotational
movement of one component results in a similar movement of the other
component.
It therefore includes entanglement means such as "hooks", (which is intended
to
include all configurations in which a protruding member interacts with a
fabric in an
entanglement mode),with cooperating fabrics having loopy surfaces such as
velours,
and interlocking molded shapes.
The support member is conveniently made from a relatively rigid plastic such
as a polypropylene, nylon, acrylonitrile-butadiene-styrene copolymer (ABS),
high-
impact polystyrene, similar dimensionally stable polymers and rubbers that are
relatively rigid such as at least partially vulcanized rubbers. It can also be
made from
a metal or a ceramic if desired though this tends to be more expensive.
The support member is provided with means adapted to permit the backup pad
to be mounted on a rotary drill or grinder. This means can comprise a spindle
which
is either integral with the support member or attachable thereto by a suitable
mechanism, The means can also comprise a socket for a drive spindle such as a
socket with snap-fit mechanism or other quick release catch, a threaded socket
or
member, or similar device.
Dra- wings
Figure 1 is a plan view of a backup pad according to the invention shown from
a first
side and
Figure 2 is a plan view of the same backup pad from the opposed side.
Description of Preferred Embodiments
The backup pad according to the invention is now described with particular
reference to the Drawings which depict a preferred form of backup pad from
both
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sides. In Figure 1 only the generally disk-shaped compressible pad, 1, is
visible. This
comprises three equal and symmetrically located gaps, 2, in the circumference
of the
pad. The design direction of rotation is clockwise, the trailing edge, 3, of
each gap is
rounded. The shape of the gap is otherwise generally parabolic, with the axis
of the
parabola at an angle to the radius of the pad such that the leading edge of
the parabola
is shorter than the trailing edge. In this particular embodiment, the surface
shown is
provided with the hook component of a hook and loop attachment means. However
any other suitable attachment can of course be used.
Figure 2 shows the other side of the backup pad and includes the support
member, 4, laminated to the compressible pad, 3. In addition to the same
parabolic
gaps in the circumference displayed by the compressible pad, the support
member has
chord segments, 5, removed from the circumference and the trailing edge, 6, of
each
gap is rounded even more pronouncedly that that of the compressible pad. An
axially
located boss, 7, is used to mount a spindle, 8, by which the backup pad can be
mounted on a rotary drill or grinder.
In use an abrasive disk with essentially the same dimensions as the
compressible pad is mounted on the attachment surface of the compressible pad.
Rotation of the abrasive disk allows vision of the surface being abraded
through the
circumferential gaps. In the event that the disk contacts a surface at an
angle to the
surface being ground, the contact will occur initially and with greatest
force, at the
radial extremities of the disk. Because at that point the disk is backed by a
compressible pad but not by the relatively rigid support member, the disk and
the
compressible pad are able to deform to absorb the impact and thus avoid any
tearing
of the abrasive disk.
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